ADP-ribosylation is the post-translational modification of a protein, resulting from transfer of the ADP-ribose moiety of NAD to a specific amino acid in the protein, thereby altering its structure and function (Williamson and Moss (1990) In ADP-ribosylating Toxins and G Proteins: Insights into Signal Transduction (Moss, J. and Vaughan, M., eds) pp. 493-510, American Society for Microbiology, Washington, D.C.). Mammalian cells contain mono-ADP-ribosyltransferases (ART) that catalyze the formation of ADP-ribose-(arginine) protein, which can be cleaved by a 39-kDa ADP-ribose-(arginine) protein hydrolase (ARH1) that releases free ADP-ribose and regenerates the unmodified protein.
In addition to mono-ADP-ribosyltransferases, mammalian cells contain enzymes that poly-ADP-ribosylate proteins. Poly-ADP-ribosylation is catalyzed by a family of enzymes termed poly(ADP-ribose) polymerases (PARP) (Ame et al. (2004) Bioessays 26:882-893), that synthesize polymers of ADP-ribose in carboxylate linkage (Ogata et al. (1980) J. Biol. Chem. 255:7610-7615; Ogata et al. (1980) J. Biol. Chem. 255:7616-7620), usually to PARP-1 (Ogata et al. (1981) J. Biol. Chem. 256:4135-4137). Multiple poly(ADP-ribose) polymerases (PARPs) have been identified in the human genome, but there is only one known poly(ADP-ribose) glycohydrolase (PARG) that degrades the (ADP-ribose) polymer to ADP-ribose. Poly-ADP-ribosylation is involved in a number of critical biological processes including DNA repair, carcinogenesis, and cellular differentiation (Diefenbach and Burkle (2005) Cell Mol Life Sci. 62:721-730; Masutani et al. (2005) Cell Mol. Life. Sci. 62:769-783; Nguewa et al. (2005) Prog. Biophys. Mol Biol. 88:143-172).
Sir2 (silent information regulator 2) family proteins are involved in gene silencing, life span extension, and chromosomal stability (Guarente (2000) Genes Dev. 14:1021-1026; Bitterman et al. (2003) Microbiology and Molecular Biology Reviews 67:376-399). In the presence of NAD, Sir2 couples protein deacetylation with formation of O-acetyl-ADP-ribose and release of nicotinamide (Imai et al. (2000) Nature 403:795-800; Jackson and Denu (2002) J. Biol. Chem. 21:18535-18544). In many biological systems, specific enzymes are believed to be involved in the degradation of small molecules that are generated in signaling cascades, and thus, in termination of their effects. Thus far, enzymatic destruction of O-acetyl-ADP-ribose has been shown only with the Nudix family (O'Handley et al. (1998) J. Biol. Chem. 273:3192-3197) of ADP-ribose pyrophosphatases (Rafty et al. (2002) J. Biol. Chem. 277:47114-47122) (nucleoside diphosphate linked to another moiety, hence the acronym Nudix) and perhaps other less selective pyrophosphatases.
Proteins capable of hydrolyzing other ADP-ribose linkages are important in the regulation of ADP-ribose metabolism, which is involved in many cellular processes including chromatin decondensation, DNA replication and repair, transcription, centrosome duplication, regulation of telomere function, mitosis, necrosis and caspase-dependent and -independent apoptosis (Bonicalzi et al. (2005) Cell Mol. Life Sci. 62:739-750; Virag and Szabo (2002) Pharmacol. Rev. 54:375-429). In addition, drugs targeting polymer synthesis and turnover can be used for treating disorders associated with excessive tissue damage or as anticancer agents, radiosensitizers and antiviral agents (Southan and Szabo (2003) Curr Med Chem. 10:321-40). Furthermore, proteins that specifically target signaling molecules in the Sir2 pathway could be used in regulating chromatin.